Inkjet printers eject drops of ink through an array of nozzles to effect printing on a media substrate. The nozzles are typically formed on a silicon wafer substrate using semiconductor fabrication techniques. Each nozzle is a MEMS (micro electromechanical systems) device driven by associated drive circuitry formed on the same silicon wafer substrate. The MEMS nozzle devices and associated drive circuitry formed on a single nozzle is commonly referred to as a printhead integrated circuit (IC).
Traditional inkjet printers use scanning inkjet printheads. These have a single printhead IC that traverses back and forth across the width of a page as the printer indexes the page along. The Applicant has developed a range of pagewidth printheads. These printheads use a series of printhead IC's mounted end to end to provide an array of nozzles that extends the entire width of the page. Instead of scanning back and forth, the printhead remains stationary in the printer as the page is fed past. This allows much higher print speeds but is more complicated in terms of controlling the operation of a much larger array of nozzles.
A print engine controller (PEC) generates the print data that is sent to the printhead IC for governing when the individual nozzles eject ink. Most printhead IC's have a shift register or latch for each of the nozzles respectively. The latch holds the print instruction (a ‘1’ bit) for its corresponding nozzle until the PEC send a fire command and the fire cycle commences, during which the nozzles with ‘1’s in the latch will eject in accordance with a predetermined sequence. Once fired, all the latches in the array are loaded with the print data for the next fire cycle.
Thermal printhead IC's are more efficient if the vapor bubble generated by heater element is nucleated quickly. Less heat dissipates into the ink prior to bubble nucleation. The heat that dissipates into the ink as the heater heats up to the nucleation temperature, does not significantly contribute to bubble generation and therefore droplet ejection. To get the bubble to nucleate more quickly, the electrical pulse needs to have a shorter duration while still providing the same energy to the heater (about 200 nJ). This requires the drive field effect transistor (FET) for each nozzle to increase the power of the drive pulse. However, increasing the power of the drive FET increases its size. This enlarges the wafer area occupied by the nozzle and its associated circuitry and therefore reduces the nozzle density of the printhead. Reducing the nozzle density is detrimental to print quality and compact printhead design.